Relativistic Runaway Electrons Avalanches (RREAs) development depends on the applied electric field and the environment's air density. This dependency controls the RREA exponential growth length ...scale. The RREA development affects the bremsstrahlung excess occurring due to the passage of charged particles through the thundercloud's electric fields, the gamma‐ray glow. We used Monte Carlo simulations to develop an empirical model showing the RREA behavior in a realistic atmospheric density profile. The new formulation shows how the density variation modulates the electron population under electric field strengths near the RREA electric field threshold. The model limits the initial RREA altitude range as a function of the electric field strength. The new model is valid between ∼0.6 and ∼18 km, covering the relevant heights to investigate the generation of ground‐detected gamma‐ray glows.
Plain Language Summary
Thunderclouds are energy sources for trespassing charged particles from cosmic rays. This extra energy gain may induce electron avalanches, known as Relativistic Runaway Electron Avalanches (RREAs), resulting in an enhanced gamma‐ray flux via bremsstrahlung, the gamma‐ray glow. Recent studies relate this enhancement to electric field strengths close to the RREA requirement. The atmospheric density variations affect avalanche development by modifying the RREA requirement, resulting in isolated avalanches by imposing limits to the avalanche's initial altitude. We show how RREAs develop in a realistic atmospheric density profile. We present a modification on the characteristic avalanche length under this condition. The initial avalanche altitude is crucial because it completely modifies the density profile trespassed by a downward electron shower. Finally, we discuss the consequences of isolated RREAs for high‐energy emissions and show that the electric field strength constrains the possible initial altitudes for the gamma‐ray glow.
Key Points
A new empirical model quantifies how electron avalanches vanish because of atmospheric density variations with ∼10% accuracy
The model limits the initial altitude of electron avalanche development for electric field strengths near the avalanche threshold
We narrow the possible gamma‐ray glow source height range with the new model which is valid through ∼0.6–18 km
We evaluate the current status of supernova remnants as the sources of Galactic cosmic rays. We summarize observations of supernova remnants, covering the whole electromagnetic spectrum and describe ...what these observations tell us about the acceleration processes by high Mach number shock fronts. We discuss the shock modification by cosmic rays, the shape and maximum energy of the cosmic-ray spectrum and the total energy budget of cosmic rays in and surrounding supernova remnants. Additionally, we discuss problems with supernova remnants as main sources of Galactic cosmic rays, as well as alternative sources.
Context. The spectrum of cosmic rays (CRs) is affected by their escape from an acceleration site. This may be observed not only in the gamma-ray spectrum of young supernova remnants (SNRs) such as ...RX J1713.7–3946, but also in the spectrum of CRs showering the Earth. Aims. The escape-limited model of cosmic-ray acceleration is studied in general. We discuss the spectrum of runaway CRs from the acceleration site. The model will also be able to constrain the spectral index at the acceleration site and the ansatz with respect to the unknown injection process into the particle acceleration. Methods. Our methods are analytical derivations. We apply our model to CR acceleration in SNRs and in active galactic nuclei (AGN), which are plausible candidates of Galactic and extragalactic CRs, respectively. In particular we take account into the shock evolution with cooling of escaping CRs in the Sedov phase for young SNRs. Results. The spectrum of escaping CRs generally depends on the physical quantities at the acceleration site like the spectral index, the evolution of the maximum energy of CRs and the evolution of the normalization factor of the spectrum. It is found that the spectrum of runaway particles can be both softer and harder than that of the acceleration site. Conclusions. The model explains spectral indices of both Galactic and extragalactic CRs produced by SNRs and AGNs, respectively, suggesting the unified picture of CR acceleration.
Aim
Lactate is produced in and released from skeletal muscle cells. Lactate receptor, G‐protein‐coupled receptor 81 (GPR81), is expressed in skeletal muscle cells. However, a physiological role of ...extracellular lactate on skeletal muscle is not fully clarified. The purpose of this study was to investigate extracellular lactate‐associated morphological changes and intracellular signals in C2C12 skeletal muscle cells.
Methods
Mouse myoblast C2C12 cells were differentiated for 5 days to form myotubes. Sodium lactate (lactate) or GPR81 agonist, 3,5‐dihydroxybenzoic acid (3,5‐DHBA), was administered to the differentiation medium.
Results
Lactate administration increased the diameter of C2C12 myotubes in a dose‐dependent manner. Administration of 3,5‐DHBA also increased myotube diameter. Not only lactate but also 3,5‐DHBA upregulated the phosphorylation level of mitogen‐activated protein kinase kinase 1/2 (MEK1/2), p42/44 extracellular signal‐regulated kinase‐1/2 (ERK1/2) and p90 ribosomal S6 kinase (p90RSK). MEK inhibitor U0126 depressed the phosphorylation of ERK‐p90RSK and increase in myotube diameter induced by lactate. On the other hand, both lactate and 3,5‐DHBA failed to induce significant responses in the phosphorylation level of Akt, mammalian target of rapamycin, p70 S6 kinase and protein degradation‐related signals.
Conclusion
These observations suggest that lactate‐associated increase in the diameter of C2C12 myotubes is induced via activation of GRP81‐mediated MEK/ERK pathway. Extracellular lactate might have a positive effect on skeletal muscle size.
ABSTRACT
3C 396 is a composite supernova remnant (SNR), consisting of a central pulsar wind nebula (PWN) and a bright shell in the west, which is known to be interacting with molecular clouds (MCs). ...We present a study of X-ray emission from the shell and the PWN of the SNR 3C 396 using archival Suzaku data. The spectrum of the SNR shell is clearly thermal, without a signature of a non-thermal component. The abundances of Al and Ca from the shell are slightly enhanced, which indicates the presence of metal-enriched supernova ejecta. The PWN spectra are well described by a power-law model with a photon index of ∼1.97 and a thermal component with an electron temperature of ∼0.93 keV. The analysis of about 11 yr of Fermi data revealed an 18σ detection of gamma-ray emission from the location overlapping with the position of 3C 396 / 4FGL J1903.8+0531. The spectrum of 3C 396 / 4FGL J1903.8+0531 is best fitted with a log-parabola function with parameters of α = 2.66 and β = 0.16 in the energy range of 0.2–300 GeV. The luminosity of 3C 396 / 4FGL J1903.8+0531 was found to be >1035 erg s−1 at 6.2 kpc, which rules out the inverse Compton emission model. Possible scenarios of gamma-ray emission are hadronic emission and bremsstrahlung processes, due to the fact that the SNR is expanding into dense MCs in the western and northern regions of the SNR.
An extreme case of electron shock drift acceleration (SDA) in low Mach number collisionless shocks is investigated as a plausible mechanism for the initial acceleration of relativistic electrons in ...large-scale shocks in galaxy clusters, where the upstream plasma temperature is of the order of 10 keV and the degree of magnetization is not too small. One-dimensional electromagnetic full particle simulations reveal that, even when a shock is rather moderate, a part of the thermal incoming electrons are accelerated and reflected through relativistic SDA and form a local non-thermal population just upstream of the shock. The accelerated electrons can self-generate local coherent waves and further be back-scattered toward the shock by those waves. This may be a scenario for the first stage of the electron shock acceleration occurring at the large-scale shocks in galaxy clusters, such as CIZA J2242.8+5301, which have well-defined radio relics.
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